fix building with libpng-1.6

--- libplot/z_write.c
+++ libplot/z_write.c
@@ -164,7 +164,7 @@
     }
 
   /* cleanup after libpng errors (error handler does a longjmp) */
-  if (setjmp (png_ptr->jmpbuf))
+  if (setjmp (png_jmpbuf (png_ptr)))
     {
       png_destroy_write_struct (&png_ptr, (png_info **)NULL);
       return -1;
@@ -444,7 +444,7 @@
 #endif
     }
 
-  longjmp (png_ptr->jmpbuf, 1);
+  png_longjmp (png_ptr, 1);
 }
 
 static void 
@@ -515,7 +515,7 @@
 #endif
     }
 
-  longjmp (png_ptr->jmpbuf, 1);
+  png_longjmp (png_ptr, 1);
 }
 
 static void 
